This invention relates to devices which detect and/or treat tachyarrhythmias (rapid heart rhythms), and more specifically, to mechanisms to distinguish among various tachyarrhythmias and to provide appropriate therapies to treat the identified tachyarrhythmias.
Early automatic tachyarrhythmia detection systems for automatic cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intra-myocardial pressure, blood pressure, impedance, stroke volume or heart movement) and/or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation.
In some pacemaker/cardioverter/defibrillators presently in commercial distribution or clinical evaluation, fibrillation is generally distinguished from ventricular tachycardia using ventricular rate based criteria, In such devices, it is common to specify the rate or interval ranges that characterize a tachyarrhythmia as opposed to fibrillation. However, some patients may suffer from ventricular tachycardia and ventricular fibrillation that have similar or overlapping rates, making it difficult to distinguish low rate fibrillation from high rate tachycardia. In addition, ventricular fibrillation may display R-R intervals that vary considerably, resulting in intervals that may fall within both the tachycardia and fibrillation rate or interval ranges, or outside both. Similarly, supraventricular arrhythmias may be the cause of high ventricular rates, or may be present during ventricular arrhythmias, further increasing the possibilities of misdiagnosis.
Presently available pacemaker/cardioverter/defibrillator arrhythmia control devices employ programmable fibrillation interval ranges and tachycardia detection interval ranges, along with measurement of suddenness of onset and rate variability. For future generations of devices, numerous detection and classification systems have been proposed. Numerous patents, including U.S. Pat. No. 5,217,021 issued to Steinhaus et al., U.S. Pat. No. 5,086,772 issued to Lanard et al., U.S. Pat. No. 5,058,599 issued to Andersen and U.S. Pat. No. 5,312,441 issued to Mader et al propose waveform morphology analysis systems for determining the type and origin of detected arrhythmias. Other patents, including U.S. Pat. No. 5,205,583 issued to Olson, U.S. Pat. No. 5,913,550 issued to Duffm, U.S. Pat. No. 5,193,535 issued to Bardy et al., U.S. Pat. No. 5,161,527 issued to Nappholz et al., U.S. Pat. No. 5,107,850 issued to Olive and U.S. Pat. No. 5,048,521, issued to Pless et al. propose systems for analysis of order and timing of atrial and ventricular events.
In the Medtronic Model 7219 devices, an arrhythmia detection and classification system generally as disclosed in U.S. Pat. No. 5,342,402, issued to Olson et al., incorporated herein by reference in its entirety, was employed, which uses both strategies together. In currently marketed Medtronic implantable pacemaker/cardioverter/defibrillators, prioritized, rule based arrhythmia classification schemes corresponding generally to those described in U.S. Pat. No. 5,545,186, issued to Olson et al. and U.S. Pat. No. 5,755,736, issued to Gillberg, et al., both incorporated herein by reference in their entireties, are employed.
Classification of detected heart rhythms is also valuable in the context of implantable anti-tachycardia pacemakers which do not include cardioversion or defibrillation capabilities, pacemakers which treat bradycardia, other implantable stimulators, implantable drug dispensers and implantable heart monitors. Examples of arrhythmia classification mechanisms employed in bradycardia pacemakers are disclosed in U.S. Pat. No. 5,282,465, issued to van der Veen, et al., U.S. Pat. No. 5,549,649 issued to Florio, et al., U.S. Pat. No. 5,792,200 issued to Brewer and U.S. Pat. No. 5,144,549, issued to Olson, et al., all incorporated herein by reference in their entireties. In the context of implantable drug dispensers, particularly those in which an anti-arrhythmic drug is delivered, accurate identification of cardiac rhythms is of paramount importance. Such drug dispensers are disclosed in U.S. Pat. No. 4,146,029, issued to Ellinwood and U.S. Pat. No. 5,527,344, issued to Arzbaecher, et al. and U.S. Pat. No. 5,220,917, all incorporated herein by reference in their entireties.
One of the most difficult problems in the design of implantable cardioverter defibrillators (ICDS) and other devices that need to be able to correctly classify cardiac arrhythmias, is the design of mechanisms to discriminate between tachycardias that should be treated and those that should not. For example, sinus tachycardia (ST) should never be treated by an ICD. Particularly in the context of dual chamber bradycardia pacemakers, a determination of whether a detected rapid atrial rhythm is a reentrant tachycardia or a sinus rhythm is valuable in determining whether the pacemaker will deliver ventricular pacing pulses in synchronism with the detected atrial rhythm or switch to a non-synchronized pacing mode. One early original mechanism for discriminating between sinus tachycardia and reentrant atrial or ventricular tachycardia was the onset criterion, as described in the above-cited Olson, et al. ""402 patent and in U.S. Pat. No. 4,880,005 issued to Pless, et al. This simple mechanism for discriminating against sinus tachycardia considered only ventricular rate information. As an improvement, the ST Rule as disclosed in the above-cited Olson, et al. ""186 patent was created. The ST Rule and the onset criterion as employed in the Olson ""186 patent are mutually exclusive in their use of timing information. The ST Rule considered only the timing of P-waves relative to R-waves whereas the onset criterion considered only the timing of R-waves relative to R waves.
The present invention is directed to a third generation adaptive xe2x80x9cNew ST rulexe2x80x9d that provides a more accurate ST detection mechanism for next generation cardiac pacemakers, ICDS, drug pumps, monitors or other devices. The New ST Rule is a rule which, when met, may control delivery or withholding of therapy. In the particular embodiment disclosed below, the New ST Rule is incorporated into a prioritized, rule-based arrhythmia classification system in an implantable pacemaker/cardioverter/defibrillator. In this particular embodiment, if the New ST Rule is met, antitachyarrhythmia therapy is withheld even if a lower priority rule determines that a treatable tachyarrhythmia is present. In other applications of the New ST rule, it might be employed as part of a mechanism to control delivery of bradycardia pacing. For example, a switch to non-synchronized ventricular pacing might be prevented in the presence of a rapid atrial rate, if the New ST Rule is met. Similarly, the New ST Rule may be employed as an arrhythmia classification mechanism in an implanted drug dispenser, used for example to prevent unneeded increases in anti-tachyarrhythmic drug delivery in response to rapid non-pathological sinus rhythms and may also be used as an arrhythmia classification mechanism in an implanted cardiac monitor.
The New ST Rule comprises an adaptive onset criterion. An xe2x80x9cexpected rangexe2x80x9d of RR intervals is calculated based on a trimmed mean and a metric of R-R variability (similar to a standard deviation), derived over a preceding series of R-R intervals. The expected range is updated adaptively after ventricular events. Preferably the expected interval range is not updated during non-1:1 rhythms, as described in more detail below. Thus, during 1:1 rhythms, the expected range of R-R intervals follows the trend of observed R-R intervals as it increases and decreases. Consistent R-R intervals result in a narrow expected range, whereas more variable R-R intervals broaden the expected range. Continued presence of R-R intervals within the expected R-R interval range is inconsistent with the rapid rate change associated with onset of ventricular tachycardia and is considered by the New ST Rule as evidence of sinus rhythm and assists in allowing the New ST rule to ultimately be met. Conversely, downward departures of the R-R intervals from the expected R-R interval range are considered as evidence of the onset of ventricular tachycardia, and correspondingly work to prevent the ST Rule from being met. The ability of the expected interval range to adapt to the underlying degree of variability of the patient""s heart rhythm improves the accuracy with which the New ST Rule distinguishes between sinus tachycardia and ventricular tachycardia.
The sensitivity of this adaptive onset criterion provided by the New ST Rule is preferably further augmented by available atrial information. First, consistent non-1:1 rhythms are treated as evidence against the presence of sinus rhythm. Care is exercised to allow for non-sustained ectopy an inappropriate atrial sensing due to the presence of far-field R-waves (FFRW). Second, the New ST Rule monitors for xe2x80x9cP-R onsetxe2x80x9d, i.e., a sudden change in the P-R interval, as evidence against the presence of sinus rhythm. An expected range of P-R intervals is established and updated adaptively in a similar manner to the R-R expected range. P-R intervals that fall outside of their expected range are treated as evidence against sinus rhythm. This xe2x80x9cP-R onsetxe2x80x9d feature is beneficial for proper treatment of VT with 1:1 retrograde conduction that does not have a significant ventricular rate onset.
The evidence for sinus rhythm in the embodiment described below is accumulated via beat codes, pattern codes and a continuous recognition machine with exponential decay, similar in general operational aspects to the ST Rule as described in the above-cited Olson et al patent. Pattern codes indicative of gradual rate changes and normal sinus rhythm will adhere to the grammar defined by the continuous recognition machine and will, if they persist, result in the New ST Rule being met. Pattern codes inconsistent with sinus tachycardia, e.g. indicative of non-1:1 rhythms or rapid rate change associated with sudden onset of VT, will not adhere to the associated grammar and will result in the New ST Rule not being met. A more detailed description of a preferred embodiment of the New ST Rule is provided below.